Dendritic cells (DCs) maintain tolerance by disabling autoreactive T helper cells and generating regulatory T cells. In systemic lupus erythematosus (SLE), tolerance is broken through aberrant activation and homeostasis of DCs. In health, DCs discern self from non-self and infected tissues via toll-like receptors (TLRs). In SLE, however, immune complexes (ICs) comprised of autoantibodies (autoAbs) and nuclear antigens (Ags), through TLRs 7 and 9, activate myeloid DCs (mDCs) and stimulate plasmacytoid DCs (pDCs) to make large amounts of interferon-alpha (IFN-a), which feeds back to enhance activation of mDCs and autoreactive lymphocytes. Female sex steroids are implicated in SLE pathogenesis because 1) nine out of ten patients are female; 2) SLE patients show increased estrogen (E) activity and decreased progesterone (Pg) and androgen activity; 3) in SLE animal models, disease is exacerbated by E; and 4) E has direct stimulatory effects on B cells and DCs. In contrast, Pg is an immunosuppressive female sex steroid. How female sex steroids, particularly Pg, regulate DC functions and lupus autoimmunity is poorly understood. We have shown that Pg suppresses TLR-induced IFN-a production by pDCs in mice, which suggested Pg could regulate lupus disease. Indeed, our new preliminary data show that continuous Pg treatment significantly decreased mortality and nephritis in lupus-prone NZB x NZW F1 (NZB/W) mice. We now hypothesize that Pg and E have opposing effects on lupus disease development through differential regulation of DC functions. To test this, we will compare the effects of E and Pg on DC cytokine production, migration, and T cell stimulation in normal C57BL/6 (B6) mice. To ask whether hormonal regulation of DC functions is operational and normal in lupus mice, we will compare DC functions in B6 mice with those in a model of spontaneous lupus on a B6 background. The B6.Sle1.Sle2.Sle3 triple congenic (B6.TC) model closely reconstitutes the NZB/W phenotype of female-predominant anti-dsDNA Abs, glomerulonephritis and mortality. We will take advantage of this model with genetic studies designed to answer two important questions: 1) are Pg effects on DCs alone sufficient to regulate lupus-like autoimmunity; and 2) does endogenous Pg regulate DCs and disease development? These proposed studies will provide critical insight into hormonal regulation of SLE disease development. Moreover, they will identify novel therapeutic approaches for treatment and prevention of disease development in SLE patients.